1. Field of the Invention
This invention relates to an in-cylinder pressure detection device mounted on an internal combustion engine to detect pressure within a cylinder.
2. Description of the Related Art
Conventionally, an in-cylinder pressure detection device of the above-mentioned kind has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2002-339793. The detection device is integrally formed with a glow plug inserted into a cylinder head of an internal combustion engine, and has a piezoelectric element. The glow plug is comprised of a housing, a heater case provided at the lower end of the housing, and a center shaft extending upward from the inside of the housing and in contact with the heater case. Further, the glow plug is mounted in the cylinder head by screwing the housing into the cylinder head such that the heater case projects into the combustion chamber. The piezoelectric element is sandwiched between the housing and a nut screwed onto the center shaft, thereby being fixed to the upper end face of the housing in a preloaded state.
In the detection device, when the heater case of the glow plug is distorted due to combustion pressure, the distortion is transmitted to the center shaft, whereby the nut fixing the piezoelectric element is displaced vertically. As a result, the preload on the piezoelectric element is changed to cause distortion of the piezoelectric element, whereby an electric signal indicative of the sensed combustion pressure is output from the piezoelectric element.
In the above described conventional detection device, when the housing is tightened by tightening torque for screwing the same into the cylinder head, it is compressed to displace the center shaft upward relative to the housing, so that the preload on the piezoelectric element is reduced. The degree of reduction of the preload depends on the tightening torque. Generally, a piezoelectric element has a characteristic that the sensitivity thereof is higher as preload is larger. Therefore, variation in the tightening torque causes a change in the preload and the resultant change in the sensitivity of the piezoelectric element, which makes the detection accuracy of the detection device unstable. Although there has also been proposed another type of detection device in which preload on a piezoelectric element is increased by tightening torque, in this case as well, the sensitivity varies with the tightening torque, which causes a similar problem.
Further, the above-mentioned center shaft is held in contact with the heater case projecting into the combustion chamber, and hence the center shaft thermally expands due to the heat of high-temperature combustion gases transmitted via the heater case. This thermal expansion also causes upward displacement of the nut pressing the piezoelectric element against the housing. Therefore, in this case as well, the preload on the piezoelectric element is reduced to change the sensitivity of the piezoelectric element, causing the same problem as described above.